The present invention relates to a four-wheel drive control system and method for controlling a distribution of a driving force between main and auxiliary driving wheels (e.g., front and rear wheels) of a vehicle by controlling a driving force distribution mechanism in accordance with a rotational speed difference between the main and auxiliary driving wheels.
It has heretofore been proposed a four-wheel drive control system that controls a driving force distribution between front and rear wheels (e.g., main and auxiliary driving wheels) by controlling a driving force distribution mechanism in accordance with a rotational speed difference between the front and rear wheels.
Such a four-wheel drive control system is adapted to judge that a slip or spin of the main driving wheels is caused to obstruct start or acceleration of the vehicle when a rotational speed difference between the front and rear wheels becomes large and increase a driving force distribution to the auxiliary driving wheels.
In the above-described four-wheel drive control system, the driving force to be distributed to the auxiliary driving wheels is not increased until the rotational speed difference between the main and auxiliary driving wheels becomes equal to or larger than a predetermined value and not decreased until the rotational speed difference becomes lower than the predetermined value, i.e., a feedback control based on the rotational speed difference is executed.
For this reason, the above-described four-wheel drive control system encounters a problem that a control hunting that will be described later occurs at running on a low friction road (hereinafter referred to as low-μ road), climbing, acceleration, etc.
Namely, when the rotational speed difference becomes large, a driving force to be distributed to the auxiliary driving wheels is increased based on the judgment that a spin of the main driving wheels is caused, for thereby making smaller the rotational speed difference between the front and rear wheels. When the rotational speed difference has become small by the above-described control, the driving force to be distributed to the auxiliary driving wheels is decreased based on the judgment that the spin has been ended. However, when the driving force to be distributed to the auxiliary driving wheels is decreased, the rotational speed difference between the front and rear wheels becomes larger again so that the driving force to be distributed to the auxiliary driving wheels is increased again. Thus, there exists a possibility of repeating the control of increasing the driving force distribution to the auxiliary driving wheels and the control of decreasing the same alternately. In such a case, there occurs a problem that the slip or spin of the main driving wheels is never ended.